It is well known in the electronics art to place a heat sink in contact with an electronic device so that waste heat generated by operation of the electronic device is thermally transferred to the heat sink to cool the electronic device. However, with continued increases in areal densities and system clock speeds in electronic devices such as microprocessors (CPU's), digital signal processors (DSP's), and application specific integrated circuits (ASIC), the amount of waste heat generated by those electronic devices and the operating temperature of those electronic devices are directly proportional to clock speed and device geometries. Efficient operation of a CPU as well as other high power dissipation electronic devices requires that waste heat be continuously and effectively removed.
However, as the aforementioned areal densities and system clock speeds of electronic devices continue to increase, a heat flux of the electronic devices also increases. Although air cooled heat sinks are commonly used to dissipate waste heat from the aforementioned electronic devices, the increased heat flux in high performance electronic devices is often concentrated in a small area, usually on a package surface that will be placed in thermal contact with the heat sink. The ability to effectively dissipate ever increasing levels of heat flux in high performance electronic devices has challenged current heat sink designs where the entire heat sink is fabricated using processes such as machining, forging, casting, and extrusion. Those processes make it difficult to increase the number of fins or an area of the fins in order to effectively dissipate heat flux concentration.
Typically, a heat mass includes a mounting surface that is in thermal communication with the electronic device and is operative to thermally conduct the waste heat away from the device and into the heat mass. As a result, the heat flux from the electronic device is concentrated in the area of the heat mass near the mounting surface. Ideally, it is desirable to spread the heat flux in the heat mass over as much of a volume of the heat mass as possible so that the heat is efficiently transferred to the fins and dissipated by the air flow over the fins.
Heat flux is a thermal output per unit of area (i.e. W/cm2). For example, if a total thermal output is 100 Watts over a heat source having dimensions of 3.5 cm*3.5 cm, then the heat flux is 100 W÷(3.5 cm*3.5 cm)=8.163 W/cm2. At present, based on area and cost constraints, electronic device package size remains the same or decreases while the areal densities and clock speeds continue to increase. Consequently, the problems associated with heat flux concentration continue to increase and those problems cannot be solved solely by increasing heat sink size, the number of fins, or fan capacity.
Heat flux concentration can be exacerbated by conditions that reduce an efficiency of heat transfer to/from the heat mass. In instances where a liquid is used to transfer waste heat to the heat mass, a tilting of the heat mass from an optimal position (e.g. a horizontal orientation) can result in the liquid being displaced with a resulting reduction in thermal transfer from the liquid to the heat mass. In some prior heat sink designs, a heat pipe in contact with the liquid is used to transfer the waste heat from the liquid to the heat mass. However, the displacement of the liquid caused by the titling can result in a reduced contact or no contact at all between the heat pipe and the liquid. Consequently, heat transfer to the heat mass is reduced when the heat sink has a non-optimal orientation.
Typically, waste heat from the heat mass is dissipated by an air flow through fins that are connected with the heat mass. However, in many prior heat sink designs, a bottom portion of the fins are placed in close proximity to a base plate that is used to mount the heat mass (e.g. mounting surface) in thermal communication with the electronic device to be cooled. The air flow passes through the fins and is obstructed by the base plate resulting in reduced air flow, turbulent air flow, back pressure, and air shock noise. Ideally, the air flow through the fins should be smooth and unobstructed so that the heat transfer from the fins and the heat mass to the air flow is optimal. Moreover, when a system fan is used to supply the air flow to two or more heat sinks, the base plate or some other structure that obstructs the air flow can significantly reduce heat transfer from the heat mass and fins to the air flow.
Furthermore, many prior heat sink designs resort to a configuration where the heat sink is mounted to an electronic device carried by a PC board or mother board with a resulting horizontal placement of the heat sink that matches a horizontal mounting of the electronic device on the PC board. However, this horizontal placement does not always allow for the aforementioned optimal air flow. Therefore, flexibility in a placement of the heat sink and its fins relative to the air flow is lacking in prior heat sink designs.
Consequently, there is a need for a cooling device with improved thermal conductivity that reduces heat flux concentration. There is also a need for a cooling device that efficiently transfers waste heat from a liquid when the cooling device has a non-optimal orientation. Finally, there exists a need for a cooling device that allows for flexibility in positioning the cooling device to obtain an unobstructed air flow through fins of the cooling device.